Valves formed from an inert, corrosion resistant thermoplastic material are well suited for use in chemical or chemical engineering systems. These corrosion resistant thermoplastic valves can be used with pipes or tubing formed from the same or similar material to accommodate and precisely control the flow of polycorrosive chemicals. For example, valves of this type often are used to accommodate and control the flow of sulfuric acid, hydrofluoric acid, nitric acid and other oxidizing chemicals as well as caustics, solvents and halogens, to name a few. Frequently, these corrosive liquids are at temperatures as high as 250.degree. F., at pressures of up to 150 p.s.i.
Valves and pipes used in corrosive liquid systems, as described herein typically are formed from a polyvinyl chloride (PVC) or a chlorinated polyvinyl chloride (CPVC). For certain uses, valves and pipes may also be formed from polypropylene or polyvinylindene fluoride (PVDF).
The chemical or chemical engineering systems in which these corrosion resistant thermoplastic valves are used often require a precise on-off sequencing of one or more valves. The valves employed to carry out these on-off functions typically are either ball valves, gate valves, butterfly valves or diaphragm valves. In many instances, the success of a particular chemical engineering operation depends upon the precise and timely on-off sequencing of a plurality of such valves. Stated differently, an incomplete or untimely operation of even one such valve in a system can cause a costly and perhaps dangerous failure of a batch or blend of chemicals.
To facilitate the proper on-off sequenceing of a plurality of valves in a system, it is known to use electrically actuated valves. These electrically operated valves employ an electric motor in place of the more widely known manually rotatable valve handles. The electric motor is in communication with the valve stem such that upon an appropriate electric signal, the motor is activated to rotate the valve stem.
The mode change from opened-to-closed or from closed-to-opened on ball valves and on butterfly valves merely requires the rotation of the valve stem through a 90.degree. angle. A rotation of something other than 90.degree. will result in a valve that is either not completely closed or not entirely opened. Thus, rotation of a valve stem on either a ball valve or a butterfly valve through an angle either less than or greater than 90.degree. could result in a continued small flow of a particular corrosive chemical during periods when that chemical is no longer required. Cponversely certain improper rotations of the valve stem on ball or butterfly valves could result in a less than optimum flow of a particular corrosive chemical when full flow is required.
Limit switches are employed in the prior art electrically operated ball and butterfly valves to control the amount of rotation of the valve stem. Specifically, a portion of the valve stem intermediate the valve and the motor defines a cam which is intended to be in communication with the limit switch of the prior art valve. The outer surface of this cam includes two opposed arc portions defining opposed arc surfaces of a common cylinder joined by two planar chordal surfaces of the same cylinder. The limit switch of the prior art valve includes a feeler that is intended to follow the arcs and chords of the cam as the valve stem rotates. As the feeler moves across the chords of the cam, the distances between the feeler and the axis of the valve stem is intended to vary, thereby causing the angular position of the feeler to vary as well. The feeler of the limit switch is operative to generate a signal in response to appropriate changes in its angular alignment. Thus, in operation, an external signal activates the electric motor of the prior art valve, and causes the valve stem to rotate. As the valves stem rotates the feeler of the limit switch follows the contour of the rotating cam. After a 90.degree. rotation, the feeler should have moved through its maximum range of movement, thereby causing the limit switch to stop the motor.
It has been found through extensive use of the above described prior art valves that systems employing these valves often yield imprecise mixtures and flows of chemicals. More particularly, it has been found that after a period of use, the prior art valves would not always stop in either the precisely opened or the precisely closed position. In certain instances the motor would cause the valve stem to rotate continuously, thereby rapidly switching back and forth between the opened and the closed position of the prior art valve. These improper operations has the potential of causing extremely costly and dangerous failures of many chemical engineering processes. It is believed that a significant proportion of the prior art valve failures have been caused by small, vibration related movements of the limit switch relative to the cam.
Accordingly, it is an object of the subject invention to provide an electrically operated valve that insures efficient and precise opening and closing.
It is another object of the subject invention to provide an electrically operated ball valve that can operate precisely in high vibration environments.
It is a further object of the subject invention to provide an electrically operated ball valve wherein relative vibrational movement between the limit switch and valve stem cam is prevented.